Instead of pruning, make an install plan for the whole environment.

Previously, we used a hack for reinstalling reverse dependencies: we created an
install plan for the sandboxed package and the modified add-source deps and then
pruned the sandboxed package from it. This missed those revdeps that the
sandboxed package didn't depend on and also broke the sandboxed package if it
was installed in the sandboxed package DB (see #1229).

This commit replaces that hack with a more principled approach: we create a plan
for the whole environment (all packages in the depResolverInstalledPkgIndex),
constraining the modified add-source deps to be reinstalled and the already
installed packages to be preferably not.

Fixes #1229.